1. Field of the Invention
The present invention relates to a satellite broadcasting receiving tuner for receiving a digital system satellite broadcasting.
2. Description of the Related Art
A conventional satellite broadcasting receiving tuner will be described with reference to FIG. 2. Initially, a digital system satellite broadcasting signal (hereinafter simply referred to as a satellite broadcasting signal) is transmitted from a satellite in the frequency band ranging 11 to 12 GHz, frequency-converted by an outdoor down-converter, not shown, into a band ranging from about 950 MHz to 2150 MHz, and then inputted to an input terminal 21 of a satellite broadcasting receiving tuner.
The satellite broadcasting signal inputted to the input terminal 21 is sequentially passed through a bandpass filter 22, a low-noise amplifier 23, a variable attenuator 24 and an amplifier 25 and inputted to a first demodulator 26 and a second demodulator 27. The bandpass filter 22 comprises a so-called variable bandpass filter and has a frequency band (about 27 and several MHz to 36 MHz) of a satellite broadcasting signal of one channel wherein a center frequency thereof is changed in response to a frequency of a satellite broadcasting signal of a desired reception channel. The variable attenuator 24 is comprised of a pin-diode, not shown, and a current flowing through this pin-diode is changed in response to the level of the received satellite broadcasting signal, thereby resulting in an attenuation amount being controlled.
The first demodulator 26 and the second demodulator 27 constitute a QPSK demodulator. A local oscillation signal (first local oscillation signal Lol) from a local oscillator 28 is directly inputted to the first demodulator 26. A local oscillation signal (second local oscillation signal Lo2) which results from delaying the local oscillation signal from the local oscillator 28 by .pi./2 (90 degrees) with a phase-shifter 29 is inputted to the second demodulator 27. Here, the frequency (local oscillation frequency) of the local oscillation signal is oscillated at the same frequency (specifically, almost center frequency of the band of the satellite broadcasting signal of that channel) as the frequency of the satellite broadcasting signal of the desired reception channel.
Then, the first demodulator 26 generates an I signal serving as a first baseband signal by mixing the inputted satellite broadcasting signal and the first local oscillation signal Lo1. The second demodulator 27 generates a Q signal serving as a second baseband signal by mixing the inputted satellite broadcasting signal and the second local oscillation signal Lo2. Accordingly, this satellite broadcasting receiving tuner directly obtains the baseband signal from the received satellite broadcasting signal, and hence comprises so-called direct conversion tuner.
The I signal outputted from the first demodulator 26 is amplified by a first baseband signal amplifier 30, eliminated in higher harmonic component by a first low-pass filter 31, and then introduced from an output terminal 32 to a circuit of the succeeding stage within a satellite broadcasting receiver, not shown.
Similarly, the Q signal outputted from the second demodulator 27 is amplified by a second baseband signal amplifier 33, eliminated in higher harmonic component by a second low-pass filter 34, and then introduced from an output terminal 35 to a circuit of the succeeding stage within a satellite broadcasting receiver, not shown.
However, in the conventional satellite broadcasting receiving tuner, a plurality of inputted satellite broadcasting signals are arranged in the band of 950 MHz to 2150 MHz. In addition, a difference between the frequency of the satellite broadcasting signal of the channel in which the frequency is lowest and the frequency of the satellite broadcasting signal of the channel in which the frequency is highest is more than 1 octave. Further, since the frequency of the inputted satellite broadcasting signal is as high as in the GHz band, it is difficult to make a transmission characteristic (so-called skirt characteristic) of the bandpass filter 22 steep. Therefore, when the satellite broadcasting signal (e.g. frequency is 1000 MHz) of the channel in which the frequency is lowest is received, the satellite broadcasting signal (e.g. frequency is 2000 Hz) of the channel in which the frequency is doubled is inputted to the first demodulator 26 and the second demodulator 27 under the condition that it holds a certain level.
On the other hand, although the local oscillator 28 is oscillated at the same frequency (e.g. 1000 MHz) as the frequency of the received satellite broadcasting signal, this local oscillation signal contains a twice higher harmonic wave (2000 MHz) in addition to a fundamental wave (1000 MHz). Therefore, although the first demodulator 26 and the second demodulator 27 output baseband signals based on the received satellite broadcasting signal (1000 MHz), in addition, the satellite broadcasting signal (2000 MHz) whose frequency is higher by 1 octave and the twice higher harmonic wave (2000 MHz) from the local oscillator 28 are mixed so that the baseband signal based on the satellite broadcasting signal in which the frequency is higher by 1 octave is also outputted. As a result, the satellite broadcasting receiver has the problem that a disturbance is caused by the baseband signal based on the satellite broadcasting signal in which the frequency is higher by 1 octave.